Atmospheric StorageTanks are an integral part of any refinery or process plant as they are used to store fluids for various purposes for short or long duration. Feedstock tanks and product tanks provide buffer capacity between process plants, which are generally in continuous operation, and the supply/dispatch of feedstock/products, which are batch operations in general. Intermediate tankage is provided where there is a requirement to buffer the flow of an intermediate from one process unit to another. Component tanks are the intermediate tanks between the process units and the blenders. These tanks are necessary to de-couple blender operation from process unit operation, in order to give the blender sufficient degrees of freedom to blend the product optimally.
Swing Tanks
In determining the requirements for feedstock/product, intermediate and component tankage, it may become apparent that some of the tanks will only be in service for a relatively small proportion of the time. In these circumstances, there is potential for specifying/designing tank(s) for more than one duty. Reducing the number of tanks can have a significant impact on capital costs. Such multi-purpose tanks are often referred to as “swing tanks”.
Tanks are frequently used at the start and finish of a process and it is, therefore, necessary to know the quantity and quality of the contents. Most fixed roof tanks are calibrated after construction, and a dip hole is frequently fitted to the top of the tank so that the depth of the liquid within can be measured accurately with dip tape. This dip hole is often used to collect samples using a specialist container that can be opened once it has reached the required depth.
Modern-level instrumentation can be very accurate and has in many cases replaced the need for regular tank dipping. However, it is good practice to retain the dip pipe both as a means of checking the level instruments and as a means of getting samples. Storage tanks can be of the following types:
Roofless Tanks
This type of tank (Fig. 1) is normally used for water services including fire water, cooling water and service water. Potable water will normally be stored in a tank with a roof. Although open-top tanks are sometimes made of steel, it is common to construct them from concrete, or glass-reinforced plastic (GRP). The main advantage of a roofless tank is its low cost.
Fixed Roof Tanks
Fixed roof tanks (Fig. 1) usually make up the majority of tanks used in a typical tank farm. The tanks are generally used for low vapour pressure materials rather than higher vapour pressure materials, as these would tend to give high VOC emissions during tank breathing and could generate an explosive mixture in the vapour space.
Fixed roof tanks
are used typically for gas oil, fuel oil, and sometimes for kerosene in cold
locations.
Tanks are very good settling vessels and any water or similar liquid that is not completely removed by the processing unit is likely to settle in downstream tanks. This can affect the material selection for the tank, the provision of drains, or the provision of mixers if it is necessary to keep the two phases in suspension.
Floating Roof Tanks
Floating roof tanks are used for higher vapour pressure materials as they reduce both evaporative losses and the concentration of explosive mixtures. However, it is essential that the vapour pressure can never reach atmospheric pressure.
The design of the floating roof can be pan, pontoon, or double skin. However, regardless of type, all floating roofs need to have a seal. These are to minimise the losses from the tanks and to prevent the ingress of rainwater into the tank. Most modern tanks are fitted with double seals to reduce VOC emissions. A scraper seal is fitted if the tank contents are prone to adhering to the tank walls. Floating roof tanks are used typically for crude oil, naphtha, gasoline, and kerosene in tropical locations.
Fig. 1: Various Types of Tanks
Internal Floating Roof Tanks
Internal floating roof tanks (Fig. 1) are becoming of increased importance as a result of tighter environmental requirements. They are more expensive than conventional fixed or floating roof tanks, but they do offer a number of advantages.
They can be used in place of a conventional floating roof tank where there is a need to exclude the possibility of air contamination. In this case, an internal floating roof tank can be nitrogen blanketed.
They can be used for floating roof applications where there are heavy snowfalls. Snow can sink a conventional floating roof. Tanks built for this reason may be provided with ample ventilation apertures in the sidewalls just below the roof.
They can be used where the exclusion of water is very important. Rainwater has been known to find its way around roof seals and leak rainwater into the tank. This type of tank is used for Jet Fuel where water should be eliminated from the product.
They can be used to reduce VOC emissions since the vapour space above the internal roof can be vented via emission control equipment.
There are some
disadvantages to IFRTs in addition to increased cost.
The vapour space above the internal roof, unless specially ventilated, will be prone to the production of explosive mixtures. This often has to be blanketed.
In practice, there are likely to be limitations in tank size (maximum diameter approximately 40 m ) resulting in an increase in the number of tanks.
In order to work in association with other facilities like vapour removal units, the tanks may need to be designed for slightly higher pressures
Dome Roof Tanks
Sometimes there is a need to store materials, such as very light naphthas, that have vapour pressures in excess of atmospheric pressure but are not sufficiently high to justify a sphere or a bullet.
API 620 allows for such instances by permitting designs up to a design pressure of 15 PSIG. Such vessels can have a variety of shapes, but the most commonly employed is the dome-roofed tank. A pressurised dome roof is always more costly to fabricate than a coned one. This is usually due to the major additional cost of the foundations. The additional pressure is likely to push out the flat floor, and a special foundation is usually required to stop the tank from deforming. Over/under pressure protection for such tanks is also more like those facilities provided for pressure vessels.
There are instances when tanks such as these are needed, but most installations avoid their use by ensuring that the light hydrocarbon streams involved are safely disposed of without storing them.
Design Standard for Atmospheric Storage Tanks
Atmospheric storage tanks are essential components in various industries, storing a wide range of liquids and gases under ambient pressure. API 650: Welded Tanks for Oil Storage is the most widely used design standard for Atmospheric storage tanks.
API 650 covers the design and construction of welded, cylindrical, atmospheric tanks. It provides guidelines on material selection, tank design, and testing to ensure the tanks can safely handle the intended contents. Key aspects include:
Material Specifications: Steel grades, thicknesses, and welding practices.
Design Requirements: Minimum requirements for tank strength, stability, and safety features.
Inspection and Testing: Procedures for ensuring tanks meet performance standards and are free of defects.
Differences Between Fixed Roof Tank and Floating Roof Tanks
The major differences between a fixed roof tank and a floating roof tank are tabulated below:
Feature
Fixed Roof Tanks
Floating Roof Tanks
Roof Type
Fixed, non-movable roof
Roof that moves up and down with the liquid level
Construction
Permanent, rigid structure
Floating structure that adjusts with liquid level
Applications
Used for a wide range of liquids including water, chemicals, and oil
Primarily used for volatile liquids like gasoline and crude oil
Ventilation
Usually requires a vent or pressure relief system
Floating roof reduces vapor space, often requires fewer vents
Vapor Loss
Higher potential for vapor loss and emissions
Reduced vapor loss and emissions due to reduced vapor space
Maintenance
Easier to inspect and maintain due to fixed nature
Requires careful inspection and maintenance of the floating mechanism
Construction Cost
Generally lower due to simpler design
Higher cost due to complexity of the floating roof system
Operational Cost
Potentially higher due to greater vapor losses
Lower operational costs due to reduced vapor emissions
Sealing Efficiency
Less efficient in sealing against vapors
Highly efficient in sealing against vapors due to the floating roof
Safety Considerations
More prone to atmospheric contamination and fire hazards
Better at minimizing fire hazards due to reduced vapor space
Flexibility in Size
Suitable for various sizes and volumes
Typically used for larger tanks with high volatility liquids
Temperature Control
May require additional measures for temperature control
Floating roof can help with temperature control by minimizing vapor space
Environmental Impact
Higher potential for environmental impact due to vapor loss
Lower environmental impact due to reduced vapor emissions
Leak Detection
Easier to detect leaks at the roof level
More complex due to the floating roof mechanism and need for specialized detection
Fixed Roof Tanks vs Floating Roof Tanks
Additional Notes:
Fixed Roof Tanks: These are straightforward in design and construction, making them less costly initially. However, they may incur higher long-term operational costs due to greater vapor losses and the need for more frequent ventilation and environmental controls.
Floating Roof Tanks: These are more complex and expensive to build and maintain but are advantageous for storing volatile liquids. The floating roof helps minimize vapor loss, reduces environmental impact, and often improves safety by limiting the amount of vapor space that could potentially ignite.
Design Pressure
Although atmospheric storage tanks are intended to operate at about atmospheric pressure, in practice they do have a design pressure, even if it is only a few millibar gauges.
Tank Breathing
When the gas enters or leaves a vapour space in a liquid storage tank, the tank is said to be breathing. Breathing can occur from a number of causes.
A rise or fall in the liquid
level.
A rise or fall in the tank
temperature (consider the effect of a rain shower on a tank that has been
exposed to the sun all day).
The escape of gases coming out
of solution, or even the ingress of gases being absorbed such as vapour
breakthrough.
The escape of gases entering
the tank from other sources.
The vents on the
tank have to be large enough to pass all coincident vapour flow, both
in-breathing and out. API RP 2000 forms
the basis for establishing tank breathing rates.
The type of vent
to be used depends on the pressure to be held in the tank. If the tank is to be held as close as
possible to atmospheric pressure, and air ingress is acceptable, then a covered
vent (to keep out birds etc.) will be used.
The only pressure generated will be from the frictional resistance of
the gas entering or leaving.
If the tank is to be blanketed, then it will probably need to operate at a slightly higher pressure so that sufficient margins can be allowed between in-breathing and out-breathing for the blanketing controls to work. It would be normal to expect a blanketed tank to be fitted with a pressure/vacuum valve (PVV) and have a controlled supply of blanket gas. Under normal operation, in-breathing by the tank will cause the blanket gas to be introduced. Should it fail, then the vacuum facility will permit air ingress on the grounds that it is better to take in the air than to collapse the tank. Out-breathing causes the pressure section of the PVV to operate. The control signal for the letdown control of the blanket gas must come directly from the tank, as the frictional pressure drop from the controller to the tank is likely to be significant in comparison with the set pressure.
When out-breathing is required to pass to some kind of treatment facility, then clearly higher pressures are required as a driving force. In these circumstances, it is normal to provide PVVs with flanged outlets.
Fixed roof tanks and internal floating roof tanks normally have a vapour space that can accumulate volatile gases and/or contain air. If explosive mixtures can be produced in the vapour space, or if it is necessary to exclude air for reasons of contamination, then these tanks will require a gas blanket. In most cases, the most appropriate blanketing gas will be nitrogen. However, occasionally fuel gas may be used. When fuel gas is to be used, it is essential to ensure there are no inadvertent releases to the atmosphere, and that any PVVs discharge to an appropriate disposal system.
A flow meter is a device that measures the flow rate of a fluid. Flow meters are flow-measuring instruments used to measure the linear, nonlinear, mass, or volumetric flow rate of a gas or a liquid. Flow meters are also known as Flow Gauges or flow measurement instruments. Accurate flow measurements of gases and liquids are required for better control and quality of industrial processes.
This article will provide the following information in brief:
Types of Flowmeters/ Flowmeter types
Operating Principle of Each type of Flowmeter
Design Features of each flowmeter.
Advantages and Disadvantages of each type of Flowmeters
Selection and Application of those Flowmeters
Types of Flowmeters | Flowmeter Types
Broadly two types of flow meters are widely used in industries:
Volumetric Flowmeters and
Mass Flowmeters
Volumetric Flowmeters
Volumetric flow meters got their name because these flow meters measure the fluid volume passing through a specific location in a set period of time. Volumetric flow meters provide an instantaneous analog, digital, or pulse output of the volumetric flow rate of the liquid or gas. Various types of Volumetric Flowmeters are available as listed below
Mass flow meters measure the fluid mass flow rate that travels through a tube per unit of time. There are two types of mass flowmeters as mentioned below
Coriolis Mass flowmeter and
Thermal Mass flowmeters
Differential Head Type Flowmeters
The difference in pressure exists between the upstream & downstream sides of a restriction in a confined fluid stream, which is related to the square of fluid velocity.
Q α √ ▲P
Where Q = Volume flow rate and ▲P = Differential pressure between taps
Fig. 1: Differential head type flowmeters
Types of Orifice plates (Fig. 1)
Concentric orifice plate: Most commonly used
Segmental & Eccentric orifice plate: Used for fluids containing suspended solids.
Tappings for the Orifice plates:
Corner taps (< 1 inch)
D and D/2 taps ( 2 to 16 inches)
Flange taps (> 16 inches)
Features of Orifice Plates
Design Pressure: No limitation. Limited by DP transmitter
Design Temperature: No limitation. Limited by DP transmitter
MOC: No limitation (Steel/ Monel/nickel/ haste alloy)
Accuracy: It varies from ±0.25% to ±0. 5% of actual flow. The accuracy of the DP transmitter varies from ±0.1% to ±0. 3% of full-scale error.
Rangeability is 3:1 to 5:1.
Upstream length/ Downstream straight length is 20 / 5
Advantages of Orifice Plates
Easily installed between flanges.
Fabrication is simple and inexpensive.
No limitations on the materials of construction, line size, and flow rate
Cost is relatively independent of pipe diameter since the cost of DPT is fixed.
No process interruption for the exchange of the DP transmitter.
Disadvantages of Orifice Plates
High permanent pressure loss & hence high energy consumption to overcome pressure loss.
Impractical for systems with low static pressure.
Measuring ranges from about 3:1 to 5:1.
Accuracies decrease with Beta ratios above approximately 0.7.
Subject to damage by water hammer and foreign objects.
Venturi Meters
A venturi tube (Fig. 2) measures flow rates by constricting fluids and measuring a differential pressure drop. In the upstream cone of the Venturi meter, velocity is increased, and the pressure is decreased. Pressure drop in the upstream cone is utilized to measure the rate of flow through the instrument. Further details of the venturi meter are provided here
Fig. 2: Figure showing Venturi meter and Annubar Flowmeter
Features of Venturimeters
Design Pressure: No limitation. Limited by DP transmitter/ pipe pressure ratings.
Design Temperature: No limitation. Limited by DP transmitter/ pipe pressure ratings
Accuracy: It varies from ±0.25% to ±0. 75% of actual flow. The accuracy of the DP transmitter varies from ±0.1% to ±0. 3% of full-scale error.
Rangeability is 3:1 to 5:1.
Upstream length/ Downstream straight length is 20 / 5
Advantages of Venturimeters
Lower head losses than orifice plates reducing the capital expenditure on pumping eqpt. / save pump energy costs
No process interruption for the exchange of the DP transmitter.
Can be used for temperature extremes
Cryogenics or High Temperatures
Disadvantages of Venturimeters
Highly expensive
Larger and heavier to handle.
Annubar Flowmeter
The Annubar flowmeter is a device to measure the fluid flow (liquid, vapor, or gas) in a pipeline. The flow is measured by creating a differential pressure. As per Bernoulli’s theorem, this differential pressure is proportional to the square of the fluid velocity in the pipeline. The annubar flowmeter measures this differential pressure which is then converted to flow rate using a secondary device.
The probe is installed in the median line as a pressure sensor.
With the flow, the probe records both the static and the dynamic pressure via the probe openings.
In the minus chamber of the annubar, lying on the opposite side, only the static pressure has any effect
The differential pressure corresponds to the dynamic pressure in the pipeline & the flow can is calculated directly.
Features of Annubar Flowmeters
Design Pressure: Upto 97 bars (38 Deg. C) / 55 bars (370 Deg. C)
Accuracy: It varies from ±1% to ±2% of actual flow. The accuracy of the DP transmitter varies from ±0.1% to ±0. 3% of full-scale error.
Rangeability is 3:1 to 5:1.
Upstream length/ Downstream straight length is 20 / 5
Advantages of Annubar flowmeters
The integral manifold head allows direct mounting of DP transmitters
Hot tapping: Insertion/ installation without system shutdown
Very low-pressure drop
Disadvantages of Annubar flow meters.
Not suitable for viscous and slurry applications
Can be used only for clean fluids.
Variable Area Flowmeters/ Rotameters
Variable area flowmeters or rotameters are one of the widely used common flowmeter types. It consists of a vertical glass or plastic tube. The size of the tube varies from top to bottom; because of this, it got its name as a variable area flowmeter. Inside the tube, there is a free-moving metering float.
A free-moving float is balanced inside a vertical tapered tube
As the fluid flows upward the float remains steady when the dynamic forces acting on it are zero.
The flow rate is indicated by the position of the float relative to a calibrated scale.
Accuracy: It varies from ±1% to ±2% of actual flow.
Rangeability is 10:1
Upstream length/ Downstream straight length is 10 / 5
Advantages of Rotameters
Simple, robust, and linear output
It does not require external impulse or lead lines.
The pressure drop is minimal and fairly constant.
Disadvantages of Rotameters
Vertical installation only.
Glass tubes limit pressure & temperature and are subject to breakage from hydraulic & thermal shock
Glass tubes eroded by undissolved solids & unsuitable for metering alkaline solutions
Metal tube meters are more expensive.
Foreign particles can accumulate around the float & block the flow
Magnetic Flowmeters
Operate on Faraday’s Law of magnetic induction.
When a conductive fluid moves in a magnetic field, a voltage is generated between two electrodes at right angles to the fluid velocity and field orientation.
The flow tube has a fixed area & field intensity so the developed voltage is linearly proportional to the volumetric flow rate.
Fig. 4: Figure showing Magnetic Flowmeters
Design features of Magnetic Flowmeters
Design Pressure: 20 BARS to 172 BARS
Design Temperature: Upto 120 deg.C with Teflon liners / 180 Deg.C with ceramic liners
A pair (or pairs) of transducers, each having its own transmitter and receiver, are placed on the pipe wall, one (set) on the upstream and the other (set) on the downstream.
The time for acoustic waves to travel from the upstream transducer to the downstream transducer (td) is shorter than the time it requires for the same waves to travel from the downstream to the upstream (tu).
The larger the difference, the higher the flow velocity.
Design Features of Ultrasonic Flowmeters
Design Pressure: 207 bars (insertion type)/ unlimited (clamp on type)
MOC: mostly in aluminum, stainless steel, plastics, Hastelloy
Accuracy is + 0.5% to + 1% of flowrate
Range ability is 15:1
Fig. 6: Positive Displacement Flowmeters and Thermal mass flowmeters
Advantages of PD Flowmeters
Good accuracy and high rangeability
Can be used in viscous liquid flow
Low to medium initial set-up cost
Require no power supply and is available in a wide variety of readout devices
Disadvantages of PD Flowmeters
Maintenance is required at frequent intervals because of the `moving parts.
High-pressure drop due to obstruction
Not suitable for low flow rate
Not suitable for fluids with suspended solids
Gas (bubbles) in liquid could significantly decrease the accuracy
Thermal Mass Flowmeter
Operates by monitoring the cooling effect of a gas stream as it passes over a heated transducer.
Gas flow passes over two PT100 RTD transducers.
The temperature transducer monitors the actual gas process temperature, whilst the self-heated transducer is maintained at a constant differential temperature by varying the current through it.
The greater the mass flow passing over the heated transducer, the greater the current required to keep a constant differential temperature.
The measured heater current is, therefore, a measure of the gas mass flow rate.
Design Features of Thermal Mass Flowmeters
Design Pressure: 1200 PSIG
Design Temperature: 176 Deg. C
Sizes: 15 mm to 1000 mm
Fluids: Clean gases
Flow range: 0 – 2500 SCFM
MOC: mostly in stainless steel/ glass, Teflon, monel
Accuracy is +1% to + 2% of flowrate
Range ability is 10: 1 to 100:1
Upstream length/ Downstream straight length is 5/ 3
Advantages Of Thermal Mass Flowmeter
No temperature or pressure compensation required
Linear output (as temperature differential is proportional to mass flow)
Can be used on corrosive process streams if proper materials are specified
DC voltage or 4 to 20 mA dc outputs are available
Disadvantages of Thermal Mass Flowmeters
Practical for gas flow only
Subject to blockage by foreign particles or precipitated deposits due to small openings in the flowmeter
Power requirements are excessive in larger pipe sizes
Has to be taken out of the process line for servicing
Accurate field calibration is difficult
Coriolis Mass Flowmeter
When a moving mass is subjected to an oscillation perpendicular to its direction of movement, Coriolis forces occur depending on the mass flow.
When the tube is moving upward during the first half of a cycle, the fluid flowing into the meter resists being forced up by pushing down on the tube.
On the opposite side, the liquid flowing out of the meter resists having its vertical motion decreased by pushing up on the tube. This action causes the tube to twist.
This twisting movement is sensed by a pick-up and is directly related to the mass flow rate
There are various factors that influence the flowmeter selection for a specific industrial process. Some of those factors are:
The fluid phase and characteristics; for example, gas, liquid, steam
Flow conditions and flow range; for example clean, dirty, abrasive, or viscous fluid.
Process design parameters; for example, pressure, temperature ranges, density, viscosity, etc.
Pipe size, material, flange pressure rating, etc.
Accuracy desired.
Material of Construction (corrosive or non-corrosive fluid).
Maximum and Minimum measurement range and turn-down rates.
Repeatability and cost-effectiveness.
Environmental considerations, if any.
Regulatory and certification requirements.
Location of the flowmeter in the piping system.
Measurement in mass or volumetric unit.
Installation of Flowmeters
Flow measurement using flowmeters is an essential activity for any industry. So, it must provide reliable and accurate data. The accuracy and repeatability of measured data, to a large extent, depend on the correct installation of the flowmeter. Some of the critical steps that must be followed during flowmeter installation are listed below:
It must be installed in the proper location.
It should not be installed where there are vibrations or magnetic fields.
The flow direction must be known before installation.
Flowmeters should be installed on a straight pipe.
Some flowmeters may need straight lengths upstream and downstream of the flowmeter. It should be maintained for accurate results.
For liquid flow applications, the downward flow should be avoided.
Ensure that the flowmeter is completely filled with fluid.
Vapor or air in liquid lines and liquid droplets in gas lines should be avoided.
It’s preferable to install a filter upstream of the flowmeter to remove solids.
If repair is required, a bypass line should be provided.
Types of Pipe Materials and Their Selection / Metallic Pipe
Piping Materials are a very important part of Piping Engineering. The success of a project and the cost to a great extent depends on piping materials. Hence, the proper selection of pipe materials plays an important role in the project economy, and it is always preferred to choose piping materials that meet all technical requirements but are cheaper. There is a wide variety of piping materials present in the market. Choosing the best and most economical pipe material for a specific service is really challenging and here comes the role of piping material engineers. They, in consultation with process engineers, decide the right kind of pipe materials. In this article, we will explore the types of piping materials and their selection philosophy.
Piping material is a broad term and is not limited to only the material of the pipe. It signifies the material of all piping components; pipes, fittings, valves, and other items. So, in a sentence, we can say that piping material means the materials of all the components that are present in a specific pipe class.
Parameters for Pipe Material Selection
As pipes and pipelines carry various liquids and gases having different process conditions, the proper selection of pipe material is very important. The main process parameters that govern the piping material selection process are:
Types of Fluid to be handled (Flammable, Explosive, Corrosive, Reactive, Volatile, or Hazardous to human skin).
In addition to the above parameters, some non-process parameters that are considered for pipe material selection are:
Cost of the pipe material
Availability
Manufacturability, Weldability, etc.
Various piping codes and standards provide a long list of acceptable piping materials under their jurisdiction. These codes provide various details about those piping materials like design rules, allowable stresses at different temperatures, etc.
Pipe Material Selection Basics
Selection of basic Piping Metallurgy and Material (viz. CS, LTCS, AS, SS, etc.) for piping specification lies with the Process/Metallurgy Engineer. The main selection is based on process, licensor, and/or intrinsic metallurgy requirements to suit process medium, like corrosion, high temperature, pressure, etc.
This basic selection shall also address special considerations for PWHT, special valve trim for NACE, corrosive services like acids, amines, etc., and hazardous services like Hydrogen, Chlorine, Phosgene, Oxygen, etc.
Piping Material Basics
Metals are rarely used in their purest form as they have low mechanical strength. Alloying helps increase its properties like strength and ductility. (Easiest eg. is adding Carbon to Iron to produce ferritic Carbon Steel). Addition of alloying elements in proper proportions along with appropriate metal processing and heat treatment results in the optimization and improvement of its mechanical properties. Alloying also helps in improving corrosion & oxidation characteristics, machinability, weldability, etc.
Complex alloyed material is also being engineered for use in aerospace programs and applications. Metallic glasses and crystalline alloys have also been developed and metal alloys are sometimes even bonded with graphite, ceramic, and organic materials as composites for wider and more complex applications
Mechanical Properties of Pipe Materials
Certain mechanical properties are also considered while selecting pipe material for a specific service. They are
Elastic range: Material returns to its original shape after the load is released
Plastic range: Material is permanently deformed even after the load is released
Yield Strength – It defines the transition from elastic to plastic phase and it establishes the limiting value at which this transition occurs
Ultimate Tensile Strength – This defines the limit to which any further addition of load under constant strain would arrest the specimen elongation or thinning and would result in its failure.
Ductility – expressed in the elongation of a specimen and its reduction in the cross-sectional area before its failure. Established by measuring specimen length before elongation and minimum diameter before failure.
Hardness – Ability of a material to resist deformation. Hardness is tested by Brinell or Rockwell Hardness tests, both of which are indentation-type tests
Toughness – Ability of a material to resist sudden and brittle fracture due to the rapid application of loads. Measured using the Charpy V-Notch test.
Fatigue Resistance – Ability of a material to resist failure or crack initiation and its further propagation under repeated cyclic loading conditions
Material Terms and Definition
Before diving into the actual piping material selection process, let’s learn some of the important terms relevant to pipe materials.
Creep Strength – Ability of a metal to withstand constant weight or force at elevated temperatures without yielding
Brittle fracture – Sudden & rapid failure of a metal due to the application of energy with hardly any deformation
Stabilization – Addition of alloying elements to prevent carbon-chromium precipitation and formation of carbides, which reduces corrosion at higher temps
Intergranular Corrosion (IGC)– Corrosion occurring at grain boundaries of metals due to the depletion of chromium by the formation of the Cr Carbide layer, after reacting with carbon, which protects from further corrosive environments. (Min 12% Cr in SS). IGC is caused by reducing acids, oxidizing acids, and organic acids
Reducing acids – In Chemistry reduction means loss of oxygen and gain of Hydrogen – examples are Hydrochloric acid, Hydrofluoric acid, and hydrobromic acid
Oxidizing acids – Oxidation (a chemical reaction between metal & Oxygen) means a gain of Oxygen and loss of Hydrogen – examples are Sulfuric acid, Nitric acid, and Chromic acid
Organic acids – are of the carboxyl (COOH) group containing hydroxide (OH) – examples are Acetic acid, Formic acid, Citric acid
Stress Corrosion Cracking (SCC) – Failure of metal through a combined action of tensile stress and chemical corrosion. SCC also depends on service temp, solution environment, exposure duration, and metal properties
High-Temperature Hydrogen Attack – Results in the degradation of Carbon and Low Alloy Steel due to depletion (decarburization) of carbon (strengthening agent) in steel due to reaction with Hydrogen at high temps, thus causing loss of strength in metal.
Hydrogen Blistering – A low-temperature phenomenon where atomic hydrogen diffuses into steel and is trapped as non-metallic inclusion, which builds up pressure and eventually bulges and blisters steel.
Hydrogen-Induced Cracking (HIC) – A phenomenon similar to hydrogen blistering but HIC occurs in pipelines operating in sour services. Hydrogen blistering and HIC can be controlled by restricting sulfur content in steel to 0.005% or 0.010% max.
Oxidation – The chemical reaction of metal and alloys with oxygen in the metal in the air, to form oxides is called oxidation. This process results in scaling.
Effects of Alloying Elements on Metallic Piping Materials
The main alloying elements that influence the properties of pipe materials and hence their selection are
Carbon (C) – More carbon means more strength and hardness but less ductility and toughness.
Phosphorus (P) – High content decreases shock resistance & ductility, making the material brittle.
Silicon (Si) – Increases high-temperature properties, making the metal more stable by increasing tensile strength without increasing brittleness when under 2%. It also resists oxidization & is used as a deoxidizing agent
Manganese (Mn) – It improves hot working characteristics by increasing hardening when combined with sulfur
Nickel (Ni) – It improves hardenability by increasing the strength and toughness of steel. Combined with Chromium it improves impact and fatigue resistance. Improves low-temperature properties. Higher nickel content improves resistance to chloride cracking
Chromium(Cr) – It is a hardening element & improves material strength at a higher temp. Improves high temp oxidation & corrosion resistance of steel
Molybdenum (Mo) – It makes the steel harder and more stable by increasing its creep resistance at a higher temp. 2% Mo in steel also reduces high temp oxidation rate
Columbium/Titanium (Cb/Ti) – Commonly used stabilizing elements to improve sustained high operating temp properties of steel by reducing carbide precipitation. SS Type 321 and 347
Typical Pipe Material Selection Criteria
Now selecting a proper pipe material is a difficult task as there is a wide variety of pipe materials available in the market. Piping materials can broadly be of three types
The usual features of Metallic Cast Iron pipe materials are given below:
Metallic Cast Iron/Ductile Iron/Malleable Iron Pipes are brittle, low-strength materials used for low-temperature applications and basic utilities like air, water, drains, etc. Low-cost material.
CI shall not be used on severe cyclic condition services, excessive heat, thermal shock
DI & MI cannot be used at a temperature below -29° C & above 343°C (ASTM A47, A536)
Austenitic DI (ASTM A 571) may be used at temp up to -196°C max but not lower
Used for highly corrosive services like acids, caustic, process limited services, etc.
CS Cement lined pipe normally used in seawater applications
Alloy Steel (AS) – Also known as Cr-Mo Steel Pipe Selection
Used for high-temperature applications in CS base like process services, superheated steam, reformer gases, etc. above 400° C design temp (ASTM A335 Gr P1, 5, 11, 22, etc.)
C-1/2Mo steel can be used bet -29°C up to 454°C design temp
Cr-1/2Mo steels can be used bet -29°C and up to 550 to 600°C
The main characteristic features of metallic stainless steel piping are
Used for high-temperature and process-critical services and for cryogenic applications
Selection governed by the process for specific service needs
ASTM A312 Gr TP 304 and 316 are normally used SS grades for pipes
The presence of 2% Mo in SS316 gives better overall corrosion resistance properties than SS304
SS316 has a higher resistance to pitting and crevice corrosion in chloride environments
Grade L series has lower C (0.035%) which improves its use for higher temp up to 1100°F (600°C), has a higher resistance to IGC and better weldability, Better mechanical strength at elevated temps & good high temp oxidation resistance up to 925°C.
Grade H – Controlled C between .04 to 0.1% & lower Ni provides improved high-temperature strength above 815° C.
Common applications of SS304 are food, steel utensils, beverages, the dairy industry, etc.
Common applications of SS316 are food, pharma, marine, medical implant steel, etc.
Grade 317 – use dictated by licensor/process
Grades 321 and 347 are metallurgically very stable in high-temperature applications because of the addition of Columbium and/or Tungsten
Impact testing is not required if C < 0.1%
Refer to ASTM 01.01 for impact test requirement for low-temperature/cryogenic services
Exotic Grades – Not commonly used – High cost!!!
Duplex SS: Grade 2205/2207 (UNS No. S31803/32760)
Cr-Ni-Mo steel
Has excellent strength & corrosion resistance, improved resistance to acids and chlorides, and good weldability (ASTM A928)
Nickel 200/201: (UNS No. N02200/N02201)
Pure Nickel-very good mechanical properties
Excellent resistance to corrosive media
Good mechanical strength at a high temperature
Good ductility at low temp, good weldability
Nickel 201 has a little more Carbon than 200 which makes its use ideal for highly corrosive caustic soda (Sodium Hydroxide) applications up to 300°C (ASTM B163).
Monel 400/500: (UNS No. 04400/05500)
Ni-Cu Alloy – ASTM B165
Good resistance to saline and acidic conditions
Ideal for high-velocity seawater/brackish water applications
High resistance to cavitation and corrosion
Monel 400 has oxidation resistance up to 550°C
Monel 500 has higher tensile strength and hardness and resists oxidation up to 650°C
Ideal for use with H2SO4 and other acids
Ideal material for valve trims
Inconel Pipe Selection
Inconel 600/601: (UNS No. N06600/06601)
Ni-Cr-Fe Alloy – ASTM B167
Excellent oxidation and scale resistance properties up to 1200°C
Excellent resistance to corrosive media
Good weldability and is resistant to chloride stress corrosion cracking
Inconel 600 is used extensively in power plants Higher Cr content in Inconel 601 offers better oxidation and carburization resistance to sulfur applications
Inconel 625: (UNS No. N06625)
Ni-Cr-Mo-Fe Alloy – ASTM B705
Excellent strength and ductility between 700°C to 1100°C
The presence of Mo further increases corrosion resistance at higher temperatures
Ideal for phosphorus acids, organic acids, seawater, boiler tubing, etc.
Good weldability
Inconel 800: (UNS No. N08800)
Ni-Cr-Fe Alloy – ASTM B407
Inconel 800H: (UNS No. N08810)
Ni-Cr-Fe Alloy – This is a solution heat-treated high carbon version of Inconel 800 with improved elevated temp properties and strength
Inconel 825: (UNS No. N08825)
Ni-Cr-Mo-Fe Alloy – ASTM B423
High resistance to sulfuric acid, phosphoric acid, solvents, reducing acids, and seawater
Cupro Nickel: C70600, ASME B466/B467, B111Is an alloy of Cu & Ni, Cu 70-95% & Ni 30-95%
Has very good corrosion resistance, especially to seawater applications
Good mechanical strength & weldability
B466 seamless and B467 welded pipes are used in marine applications like seawater piping and fittings
B111 used in exchanger/condenser tubes
Also used extensively in the automobile industry, especially for brake tubing
Hastelloy C276: (UNS No. N10276)
Ni-Cr-Mo-W Alloy – ASTM B619/622
Best alloy for extremely corrosive conditions
Good for reducing and oxidizing applications
The presence of Tungsten (W-Wolfram) imparts excellent resistance to strong oxidizing services, hot contaminated acids, solvents, chlorides, etc.
Ideal for strong acids, formate acids, acetic hydride solutions, seawater, and saline solutions.
Corrosion-resistant to wet HCL, hydrochloride solutions, etc.
Alloy 20: (UNS No. N08020)
Ni-Cr-Mo-Cu Alloy – ASTM B464
Excellent mechanical properties, strength, and machinability
Excellent stress corrosion resistance to boiling 20-40% Sulfuric acid
Widely used in 98% Sulfuric acid service for valve trims
Ideal in food & pharma applications where product purity has to be guaranteed
Aluminum and Al Alloy Pipe (ASTM B345)
The main features of Aluminum and Aluminum alloy metallic piping materials are:
Excellent mechanical properties like strength, weldability, and formability
Good surface finish
High corrosion resistance
Used in Gas & Oil transmission and distribution piping systems
Also used in aircraft applications as it is light but strong
Metallic Titanium piping materials have the following features:
These are practically pure grades of Titanium with about 6% Nitrogen
Has high strength, impact toughness, fabricability, formability, and weldability
Exceptional corrosion and erosion resistance which allows
Zero corrosion allowance
Used a lot in aerospace applications and engine components besides chemical, marine, refinery, chlorine, food processing, and pharma applications
Non-Metallic Piping Materials
The most common non-metallic piping materials are PVC, UPVC, CPVC, PP, LDPE, HDPE, PVDF, HALAR, PTFE, PFA, FRP, RTRP, etc.
Polyvinyls as Pipe Materials
(PVC, UPVC, CPVC) ASTM D1784/1785
Polyvinyl Chloride (PVC) is the most common material for plastic pipe
Unplasticized (U-PVC) is the same as PVC but has resins added as additives to make it harder
Service temp up to 60°C (140°F)
Joining methods are solvent welding, threading, or flanging
Chemically inert, good corrosion and weather resistance, high strength, good electric and thermal insulator
Used in chemical processing, chilled water distribution, chemical drains, etc.
Not suitable for oxidizing agents like concentrated sulfuric acid, nitric acid, esters, and amines
Polyvinyls will burn but do not support combustion because of their high chlorine content and will extinguish immediately upon flame removal
Chlorinated PVC (CPVC) has added chlorine which makes it suitable for use at higher temps up to 100°C (210°F) and offers better corrosion resistance to liquids
Used for hot process piping, corrosive liquids, hot and cold water lines, etc.
Joining methods are solvent welding, threading, or flanging
Polyolefins (PP, CPP, PE) as Pipe Materials
Polypropylene (PP) is the lightest thermoplastic piping material. Good strength and chemical resistance, resistant to sulfur-bearing compounds. May be used up to 80°C (180°F) applications. Excellent material for industrial drainage, petroleum industry, salt water disposal, chilled water, and demineralized water lines.
Copolymer Polypropylene (CPP) is a copolymer of propylene and polybutylene. Has excellent dielectric and insulating properties, high chemical resistance, toughness, and strength between freezing to 93°C (200°F) operating temps. Excellent abrasion resistance and good elasticity. Joining by socket fusion or butt welding.
Non-metallic Polyethylene (PE) Piping Material has four classifications:
Low-density polyethylene (LDPE) has more branching and less compact molecular structure, Lower mechanical strength than polyethylene; Ideal for food handling services, brine tanks, etc. Good for temps up to 60°C (140°F). Joining by hot gas welding. All polyethylenes have excellent chemical resistance to a wide range of common chemicals.
Medium Density Polyethylene (MDPE) is a thermoplastic that has a lesser density than HDPE. It has good shock and drop resistance and better stress cracking resistance than HDPE. It has a lower hardness and rigidity when compared to HDPE. Used a lot in gas piping and fitting and in packaging films. Joining by hot gas welding.
High-Density Polyethylene (HDPE) has minimal branching and a more compact molecular structure. More rigid and less permeable than LDPE. Good for temps up to 71°C (160°F). Used for abrasion-resistant piping, caustic storage tanks, control tubing, etc. Joining by hot gas welding.
Cross-Linked High-Density Polyethylene (XLPE) is a 3-dimensional polymer of extremely high molecular weight and close molecular structure. Superior resistance to environmental stress cracking and very high impact strength. Good for temps up to 71°C (160°F). Ideal material for large storage tanks for outdoor service
Polyvinylidene Flouride (PVDF) is a strong, tough, abrasion-resistant fluoroplastic material. Resists distortion and retains strength up to 135°C (275°F). Ideal for handling wet and dry chlorine, bromine, and other halogen services. It also withstands most acids, bases, and organic solvents. PVDF is not recommended for strong caustics. The best material for high-purity piping such as deionized water. Joining methods are thermal butt, socket, or electrofusion.
Halar (ECTFE) Ethylene Chlorotrifluoro Ethylene is a very durable copolymer of ethylene and chlorotrifluoroethylene. Excellent resistance to a wide variety of strong acids, chlorine, solvents, and aqueous caustics. Excellent abrasion resistance, electric properties, low permeability, and temperature capabilities from cryogenic to 170°C (340°F). Resistant to radiation. Halar has an excellent application for high-purity hydrogen peroxide application. Joining by thermal butt fusion.
Polytetrafluoroethylene (PTFE) PTFE offers the most unique and useful characteristics of all plastic materials. PTFE can handle liquids or gases up to 232°C (450°F). PTFE flows and is used as an excellent sealant material. Normally an opaque white material.
Fluorinated Ethylene Propylene (FEP) This fluoroplastic was invented by DuPont. It can be melt-extruded and fabricated by conventional methods which allow more flexibility in manufacturing. Excellent dielectric and chemical resistance properties similar to PTFE. Use limited to temp between -54°C (-65°F) to a maximum of 150°C (300°F). It has a glossy surface and is transparent when in a thin section. FEP is widely used for its high ultraviolet light-transmitting ability.
Perfluoroalkoxy (PFA) is Similar to PTFE and FEP. Better properties than PTFE permits conventional thermoplastic molding and extrusion. Good flexibility for tubing purposes. Higher mech strength up to 260°C temperature.
Acrylonitrile Butadiene Styrene (ABS) is A family of engineered thermoplastics with a range of performance characteristics. Acrylonitrile imparts chemical resistance and rigidity to this thermoplastic. Butadiene endows it with impact strength and toughness. Styrene contributes to ease of processing. Good for hostile environments like esters, ketones, alcohols, and hydrocarbons up to 93°C (200°F).
Sulfone Polymers These are clear thermoplastics used in corrosive environments. Temp range up to 150°C (300°F). High resistance to acids, alkali, and salt solution. Not suited for ketones, chlorinated hydrocarbons, and aromatic hydrocarbons. Used a lot in flow meters and sight gauges
Forms of Sulfone Polymers
VITON – A fluoroelastomer compatible with many chemicals at varied temp ranges. Used for sealing valves, pumps, and instruments. Excellent for mineral acids, salt solutions, chlorinated hydrocarbons, and petroleum oils. The maximum temp limit is 120°C (250°F).
EPDM – Is a terpolymer elastomer made from the ethylene-propylene-diene monomer. Good abrasion and tear resistance. Excellent chemical resistance to a variety of weak acids and alkalies. Not recommended for applications involving petroleum oils, strong acids, or alkalies. The maximum temp limit is 100°C (212°F).
Nitrile BUNA-N- Nitrile rubber is a copolymer of butadiene and acrylonitrile. It has excellent elastomeric properties. It has excellent resistance to aliphatic hydrocarbons and aromatic solvents. Excellent material for valve seating. The maximum temp limit is 100°C (212°F).
HYPALON – This is a DuPont registered name for its elastomer of chlorosulfonated polyethylene. Used widely for valve seats and seals. The maximum temp limit is 100°C (212°F).
NEOPRENE – is a chlorinated synthetic rubber used primarily as a seating and sealing surface for valves. The maximum temp limit is 100°C (212°F).
NATURAL RUBBER – IS polymer isoprene with the highest molecular weight. Derived from Hevea (Rubber) tree. Used as the diaphragm and sealing material because of its elastomeric properties and resistance to abrasion. The maximum temperature limit is 100°C (212°F).
Thermoset Plastics as Piping Materials
Fiberglass Reinforced Plastic (FRP) is a highly valuable engineering material for piping and vessels (epoxy glass fiber). Very vast industrial use because of low initial cost & low maintenance. The broad range of chemical resistance. High strength-to-weight ratio. Ease of fabrication and flexibility in design. Good electrical insulation properties. It can be used up to a temperature of 150°C (300°F). Such epoxy piping is commonly used in oil, mining, and chemical industries. Sometimes used for steam condensate systems. Used in industrial cooling towers. Also used in Chlorine gas, chlorine water, and brine services.
Reinforced Thermosetting Resin Pipe (RTRP) Vinylester resin epoxy-based thermosetting resin material cured by free polymerization. Better tensile strength, elongation, and fatigue resistance. Material has excellent alkali resistance of the epoxy and acid and oxidation chemical resistance of the polymer. Used a lot in large bore seawater piping. Non-metallic piping material use is generally restricted to about 120°C (250°F). Check with vendors for specific applications for services and corrosion, sizes, fitting dimensions, jointing procedures, etc.
Typical Pipe Material Service checks
Caustic :
Always check NaOH or KOH concentration before material selection
Use Baume’s scale (Caustic Cracking Curve RP0403) for reference
The typical material is CS and PWHT is required
The typical valve trim is Monel
If PWHT is not mandatory, do not allow steam out
Amine:
Services include MEA (Mono Ethanolamine), DEA (Di Ethanolamine), DIPA (Di iso propanolamine), DGA (Di Glycolamine), etc.
The typical material is CS and PWHT is required
Fresh amines do not cause SCC; PWHT is not required, but if exposed to lean and rich amines, they require PWHT
Restrict fluid velocity to < 2 m/sec for CS
The preferred valve trim is SS 316
Use SS for higher velocity and temps
Wet H2S:
If H2S concentration in water is > wt% 50 to 75 ppm, it is susceptible to SCC
PWHT required and material hardness to be restricted to 200 BHN
NACE MR-0103 valves are not required but acceptable. Many use NACE valves as a norm
CS with 300 series SS trim is typical for valves
Always consult client/process for specific metallurgy requirements
Hydrogen (API 910/941):
Hydrogen service is defined as a combination of H2 partial pressure and temp above the curve for CS per API 941
Use Nelson’s Curve for material selection limits
CS can be used up to 232°C
232° to 330°C use 1.25Cr or 2.25Cr/Mo steel or Duplex steel
330° to 400°C use SS 321
400°C use SS 347 or Alloy 800
For higher pressure ratings (Class 600 & higher) valve casting inspection by radiography is a must and may need to go for non-standard valves – long lead item!!!
Check for PWHT requirement for piping & NACE requirement for valve trim based on pp of Hydrogen and wet H2S concentration
Hydrofluoric Acid (HF) – Concentration > 1ppm:
Very critical and hazardous service
The typical material is CS
PWHT is normally required, restrict hardness to 200 BHN
Use special precaution during the purchase of piping components, shielding, and color coding
Strictly follow piping specifications
Chlorine (Dry and Liquid):
Follow Chlorine Institute guidelines for material selection and design. Leakage is hazardous
Strictly follow pipe specification requirements
Ball valves with soft seats are typically used
CS is normally used in liquid chlorine services
PWHT is normally not required
Exercise high precaution in purchase spec for piping components w.r.t. cleanliness, packing, and shipping requirements
Ammonia (Aqueous and Gaseous application):
In aqueous (Ammonia with water), CS is a commonly used material
Strictly follow pipe specification requirements
In gaseous service, low-temperature and cryogenic materials and codes shall be used
Do not use CS if the temp is < -29°C
Oxygen (For concentration > 31%):
CGA (Compressed Gas Association) standard shall be followed
Strictly follow pipe specification requirements, special notes for design, and material selection
Commonly used piping materials are CS, SS, and Monel
Velocity in a pipe is a critical factor for material selection
PTFE is the preferred material as gasket filler and for valve stem packing
Avoid threaded piping
Cleanliness requirement is very critical during material handling/shipping after fabrication
Common Metallic Piping Materials
The widely used Metallic Piping Materials are as follows:
The most widely used seamless carbon steel pipe for high-temperature service up to 4270C is A106-B.
The most widely used seamless low-temperature carbon steel pipe for a temperature up to -460C is A333-6.
Common metallic pipeline materials for liquid and gas transport are API 5L, Gr B, X42, X52, and X60.
Cathodic Protection: Definition, Working Principles, Types, Design, Advantages, Applications
Cathodic Protection is an industrial technique for controlling metallic corrosion. Cathodic protection is commonly used on buried and submerged metallic structures like pipelines, underground storage tanks, locks, subsea equipment, offshore floaters, harbors, and ship hulls. Corrosion is an electrochemical process that operates as an electrical circuit. Corrosion occurs in the anode by oxidation and metal is lost; whereas in the cathode, protection occurs through the reduction reaction. In the cathodic protection technique, the concerned metal is converted into the cathode of the electrochemical corrosion cell to reduce corrosion. In this article, we will explore the basics of cathodic protection principles and industry practices.
Basics of Corrosion
What is Corrosion?
Corrosion can be defined as the degradation of a metal by its electrochemical reaction with a non-metallic matter such as oxygen, sulfur, etc. This occurs by forming an electrical circuit through the exchange of electrons. A corrosion cell consisting of an anode, cathode, electrolyte, and metallic connection between the anode and cathode is formed. So, for corrosion to occur, all the above four parts must be present which creates a closed circuit condition. At the surface of the metal that is exposed to the electrolyte, the electrochemical reaction occurs. The anode corrodes due to an oxidation reaction.
What is an Anode?
This is the most important part of the corrosion cell where corrosion occurs. The anode is defined as the point where the electricity is passed from the metal surface to the electrolyte by chemical means. This chemical reaction is characterized by the metal losing an electron and combining with another element, usually, oxygen is known as the oxidation reaction. In the case of steel, the resulting material is iron oxide popularly known as rust.
What is a Cathode?
The cathode is the second necessary part of the corrosion cell where protection occurs. The cathode can be defined as the point where electricity is passed from the electrolyte to the metal surface by chemical means. This is a reduction reaction characterized by the metal passing electrons to the electrolyte.
Depending upon the electrical potential compared to the other electrode, An electrode can behave either as an anode or as a cathode. This electrical potential difference is the driving electromotive force of the cell and is the voltage difference between the anode and the cathode. The electrode which is more electrically active, or more negative in voltage, undergoes the corrosion, so by definition is the anode. On the other hand, the electrode that is more noble (less negative in potential) passes electrons to the electrolyte (reduction reactions) and by definition is the cathode and does not undergo corrosion.
What is an Electrolyte?
The electrolyte, the third part of the corrosion cell, is the location where ions flow and can be any material in contact with both the anode and the cathode. The electrolyte allows oxidation and reduction reactions to occur and includes the source of atoms required for ion transfer to and from the anode and cathode.
Metallic Path: The final part of the corrosion cell is the metallic path that completes the circuit and allows the electrons to flow. It can be any metal in contact with both the anode and the cathode. For example, In the case of a tank or pipeline, the tank or pipe itself can act as the metallic path.
Fig. 1: Examples of a Corrosion cell
Effect of Corrosion
One Ampere of current can cause a loss of 9 Kg of Steel in One Year.
Even a small amount of Current discharge, 1 mA can result in 7 holes of ¼” diameter in a 2” steel pipe of standard thickness in 1 year time.
Cathodic protection is the use of DC Current from an External Source to oppose the discharge of corrosion current from anodic areas of the structure. It minimizes the potential difference between anode and cathode and in turn reduces corrosion. On the application of enough current, the whole pipeline or structure will be at one potential; thus, avoiding the creation of anode and cathode sites.
Principle of Cathodic Protection
The principle of the Cathodic Protection system is determining the anode in a large corrosion cell for making the intended material as cathode overcoming smaller corrosion cells. This can be achieved by any of the following two methods:
Galvanic Cathodic Protection:
This method first selects a more active metal from the galvanic series. Then the metal is installed in the electrolyte and a metallic path is provided. This method of cathodic protection is called sacrificial cathodic protection, or galvanic cathodic protection. In this method, a galvanically more active metal is installed to act as the anode which sacrifices itself and protects the pipe/structure working as a cathode.
Fig. 2: Galvanic Cathodic Protection
Galvanic cathodic protection systems are fundamentally very simple. The anodes in sacrificial anode cathodic protection systems need to be inspected periodically and must be replaced when consumed. The simplest systems consist of the selection of an anode fabricated from an active metal (normally, zinc, aluminum, or magnesium). Then that sacrificial anode is directly connected to the structure exposing it to the same environment as the structure/ pipeline to be protected.
Fig. 3: Galvanic Anode System
However, for buried pipelines, the anodes are not directly attached to the structure. They are evenly distributed at a shorter distance from the pipeline. Then the pipeline is connected to the anode using a wire normally through a test station.
Cathodic Protection with Galvanic Anodes:
Magnesium Anodes
Zinc Anodes
Aluminum Anodes
Advantages of Galvanic Anode Cathodic Protection System
Simple in Installation.
No External Power Source.
Very few operation or maintenance requirements.
No Power Bills.
Easy to Design.
No expensive accessories like cables etc.
Economical for small structures.
Limitations of Galvanic Anode Cathodic Protection System
Low Driving Voltage.
Poor performance due to passivation.
Limited Current. An extremely small current is available in higher-resistivity electrolytes.
Low life.
Typical Application of Galvanic anode System
Small Pipelines with good Coating.
Harbor Facilities, Steel piles, Jetties, etc.
Vessels, Tanks, etc.
Plant facilities and Equipment, Seawater intakes, Screens, Condensers, Heat Exchangers, etc.
Impressed current cathodic protection:
In the case of impressed current cathodic protection, a source of DC current is installed in the system which provides additional energy to force the current flow from an installed anode to the pipe or structural material making it a cathode. DC source can be a solar cell, rectifier, generator, battery, or some other DC power.
The anode material is selected considering the cost and weight loss per ampere year of current. Graphite, high silicon cast iron (HSCI), platinum, or mixed metal oxide are used as anodes for impressed current cathodic protection systems as they are slowly consumed. The anodes should be periodically inspected and replaced if consumed.
Fig. 4: Impressed current cathodic protection system
Cathodic Protection with Impressed current Anodes:
High silicon chromium cast iron Anodes
Mixed Metal Oxide Anodes
Graphite Anodes
Advantages of Impressed Current Cathodic Protection System
Current and Voltage can be varied.
Can be used in almost any resistivity Environment.
Can be designed for remote monitoring and control.
Can be designed for the measurement of Instant OFF / ON.
No limitation on driving Voltage.
Economically feasible to replace the anode system when required.
The system is extremely flexible.
Limitations of Impressed Current Cathodic Protection Systems
Regular monitoring and maintenance required
Requires Main supply or another source of electric Power
Interference Problems must be considered.
Relatively large chance of premature failure or breakdown.
Theory of Cathodic Protection
Steel in soil/water Anodic
Earth / Sea water Cathodic
As a result:- Steel loses electrons and hence corrosion
To reverse the above:
Make steel Cathodic
Seawater / Earth Anodic
And this is the theory of cathodic protection where the intended material is converted into a cathode.
How does Cathodic Protection Work?
Direct Current is forced to flow from an external source to the Structure. When the flow of this current is so adjusted to overpower corrosion current discharging from all anodic areas thereby providing complete Protection.
Criteria for Cathodic Protection
Steel in Soil: Pipe to Soil potential must be between –0.85 V to -1.2 V with respect to Cu/CuSo4 Reference Electrode.
Steel in Water: Pipe to Electrolyte potential must be between –0.8V to –1.10V with respect to Ag/AgCl Reference Electrode.
BASIS FOR CURRENT DENSITY….
SOIL RESISTIVITY
>1000 ohm-cm 10 mA/m2
100-1000 ohm-cm 20 mA/m2
<100 ohm-cm 35 mA/m2
WATER RESISTIVITY
>150 ohm-cm 50 mA/m2
50-150 ohm-cm 75 mA/m2
<50 ohm-cm 110 mA/m2
Fig. 5: Typical Anode and Monitoring
Cathodic Protection System Design
Prior to deciding the type of cathodic protection system and its design, certain preliminary data must be collected.
Data required for deciding a Cathodic Protection System
The following data are required for deciding on a cathodic protection system
Details of Structural Dimensions and drawings
Surface Coating Scheme
Details of Soil Strata / Terrain
Presence of Foreign Metallic Structures.
Details of cased crossings
History of corrosive areas
Stray current conditions
Operating Temperature
Availability of AC Power
Corrosion history of structures in the area
Electrolyte resistivity and pH survey report to understand corrosion rate
Planning a Cathodic Protection System
Factors that govern the Cathodic Protection System Design:
Choice of Cathodic Protection system
Amount of Total Current to achieve Cathodic Protection
No of Cathodic Protection Installations
Spacing between them &
Current Output of each Installation
Type of anodes and ground bed configuration
Any special conditions at certain locations needing modification of the general CP Plan
Location of CP Test station.
Selection of a Cathodic Protection System
The size of the structure, soil resistivity, and past corrosion history dictate the selection of a cathodic protection system. A current requirement test to protect the structure is performed before the selection. In general, when the soil resistivity is low (< 5000 ohm-centimeters) and the current density requirement is low, a sacrificial anode cathodic protection system is selected. However, for large structures with larger current requirements, a properly maintained impressed current system is used.
Cathodic Protection System Design Sequence
The following flowchart provides the design sequence for the cathodic protection system design sequences.
Fig. 6: Cathodic Protection System Design Sequence Flowchart
The below-mentioned steps are usually followed while designing a sacrificial anode cathodic protection system design:
Review soil resistivity
Select anode: For extremely corrosive soil (<2000 ohm-centimeters), Zinc is used. Otherwise, magnesium is used.
Calculating the net driving potential for anodes.
Finding the number of anodes needed to meet ground-bed resistance limitations.
Estimating the number of anodes for the system’s life expectancy.
Selecting the number of anodes to be used
Selecting the ground-bed layout
Calculation of the life-cycle cost for a proposed design.
Preparing plans and specifications
Similarly, the below-mentioned steps are followed for designing an impressive current cathodic protection system design:
Review soil resistivity
Review current requirement test
Select anode: Anode selection is based on cost consideration.
Estimating the number of anodes needed to satisfy the manufacturer’s current density limitations
Calculate the number of anodes needed to meet the design life requirements.
Calculate the number of anodes needed to meet maximum anode ground-bed resistance requirements.
A select number of anodes are to be used.
Select the area for placement of the anode bed.
Determine total circuit resistance.
Calculate anode ground-bed resistance.
Calculate ground-bed header cable resistance.
Calculate structure-to-electrolyte resistance.
Calculate total circuit resistance.
Calculate rectifier voltage.
Select a rectifier.
Calculate system cost.
Prepare plans and specifications.
Why do we need Cathodic Protection in Plants
Initial Investment for Petrochemical complexes, Fertilizer Plants, and Refineries are very high.
The corrosion problems are not detected until some leak appears.
Leakages can be extremely disastrous causing fatal accidents and great financial loss.
Increases maintenance cost of repairs of leakages.
The corrosion problem can also cause plants to shut down thereby losing production.
Cathodic Protection can be installed at the time of the erection of the plant at a very low cost.
The CP cost could be 5-6 times if it is installed after completion of the project as it involves a lot of excavation and restoration of structures.
Complexities of Plant Cathodic Protection
Scattered Underground Pipelines
-Pipes in Parallel
-Bifurcations
-Closely grouped network
Different pipes could be of different materials and coatings.
The area of influence shall vary from pipe to pipe.
Heavy underground civil structure reinforcements
Earthing network.
Tank Bottoms
Corrosion Control for Above Grade Storage Tanks using CP system
The Tank bottom in contact with soil undergoes corrosion. This could lead to leakage, loss of product, and cause environmental hazards.
It is much simpler and economical to install a CP system during the construction stage.
Cathodic Protection for Pipelines
The main data required for deciding a Cathodic Protection System for pipelines are
Details of Structure Dimensions
Surface Coating Scheme
Details of Soil Strata / Terrain
Presence of Foreign Metallic Structures.
Details of cased crossings
History of corrosive areas
Stray current conditions
Operating Temperature
Availability of AC Power
Equipment used for Cathodic Protection System
Anodes
MMO Anodes
Silicon Iron Anodes
Graphite Anodes
The anodes are installed in deep well borehole ground beds.
Fig. 7: Typical Deepwell installation with MMO Anodes
Solar Generators
In corrosion Protection Stations where the input power is not available, Solar generators are used to power the CP system.
Anode Junction Box (AJB)
The anode junction boxes are used to distribute the DC power from the T/R unit to the anodes at each CP station. The anode tail cables and the positive header (from T/R) are terminated inside the AJB. One circuit with suitably rated shunts for monitoring the current output of the anode string.
Negative Distribution Box (NDB)
The negative header cable (from T/R) and negative drain cable from the pipeline are terminated inside the NDB at each CP station. Each circuit shall have a variable resistor to control the current and suitably rated shunt to monitor the current.
Solid State Polarization Cell
Pipeline to be earthed at all overhead powerline crossings where the voltage is greater than 100 kV. Connected across the pipeline and the system earthing. Prevents the CP current drain from the structure to the system earthing. Shunts all fault currents and induced currents to the earth.
Surge Diverters
Surge Diverters are used across Isolating Joints Facilities. Provides surge protection to Isolating Joints in case of a lightning strike or other faults.
Monitoring of the Corrosion Protection System
For routine maintenance and monitoring of the effectiveness of the CP system, the following test facilities are provided along the pipeline.
POTENTIAL MEASUREMENT FACILITY: For measurement of the pipe-to-soil potential with respect to a portable reference cell. These facilities are provided at a regular interval of one facility every two kilometers
DRAIN POINT TEST FACILITY: At each CP station a drain point test facility is provided. Two permanent reference cells, two polarization coupons, and one corrosometer probe are used to measure the pipe-to-soil potential with respect to a permanent buried Cu/CuSO4 reference cell and the instant OFF potential from a buried coupon. Corrosometer allows monitoring of the corrosion rate and thereby the performance of the system. Permanent Reference cells are used to feed data for the SCADA system.
FOREIGN SERVICE CROSSING/PARALLEL FACILITY: To carry out interference testing at foreign pipeline crossing or foreign pipelines running parallel to SGP. To facilitate the mitigation of Interference effects
Cathodic Protection System Monitoring Frequency
Economic and safety considerations require close supervision and maintenance of all cathodic protection systems. Monitoring can be divided into three categories:
Monthly Monitoring
Quarterly Monitoring
Annual Monitoring
Monthly Monitoring:
Recording drain point potentials
Ground bed resistance
Anode current output
T/R output and T/R settings
Solar output settings
Quarterly Monitoring:
Recording Structure to Electrolyte Potential at the measurement location. (Only ON potentials are to be recorded)
Bonding Currents to be measured.
Isolation joints are to be tested using the Swing Test.
Reports to be generated.
Solar output settings
Close Interval Potential (CIP) and Direct Current Voltage Gradient (DCVG) Surveys
It is recommended that a Close Interval Potential Survey should be conducted within one year after commissioning the Corrosion Protection system. This should be repeated once every three to five years.
Based on the CIPS data, the DCVG survey should be conducted immediately at sites where under protection has been observed during the CIP survey. It is also recommended that a DCVG survey is conducted for the entire pipeline once every three to five years.
So, we can conclude that cathodic protection is a very useful and widely used method of steel protection. However, this method is costly and requires periodic maintenance and replacement. Click to know more about the Design of Cathodic Protection for Duplex Stainless Steel (DSS) Pipeline
The scope of piping design refers to the entire design process of connecting the equipment using appropriate piping components as determined by the process engineering team. It’s comparable to the human body; piping is equivalent to the veins that connect the main organs together.
The basic elements of Piping Design
The main aim of the piping design is to lay equipment, piping, and other accessories meeting the relevant standards, statutory regulations, and safety requirements. Of course, there are other aspects as described below.
Cost: Economic layout (equipment/ piping), optimization of materials
Constructability: Transporting and erecting large equipment, space for fabrication, the sequence of construction
Others: Weather conditions, Wind Direction, Location, Relevant regulations, Product in/out, Utilities and drain location, Conditions around battery limit, Area for future expansion
First Piping Engineers produce Site Plan, then they develop the overall Plot Plan and Unit Plot Plans. After Unit Plot Plans are in place, They develop major piping in and around equipment. They do plan for pipe racks as well.
Pipe routing studies and Nozzle /platform/ladder orientations are developed.
Underground piping is given priority as the construction starts from underground.
Based on the size, temperature, pressure, flow type, and equipment type to which the line is connected, we identify lines to be Stress Analyzed. We perform Stress Analysis using software like CAESAR II, AutoPIPE, etc.
Pipe Routing Studies
Equipment Nozzle Orientation:
Underground Piping
Underground piping refers to pipes that are buried underground and it aims to optimize the design and construction processes of a plant by finding the most effective connection between each process and different utilities.
The rise or fall in pressure caused by changes in the volume of fluid must be analyzed and applied precisely to piping design. In general, such changes in water pressure are generally known as water hammer but to be precise, hydraulic transient would be more correct. Such changes in pressure produce noise, vibration, shock, etc., which can result in the breakdown of the equipment and piping.
3D Model: Graphic data of various structures, equipment, piping, cables, and building parts of a plant displayed in virtual space using 3D CAD.
Model Review: A process carried out in-appropriate design stages to examine whether the developing work is meeting the requirements of the client and recommended design standards. It is also done to understand and provide solutions for problems in the design.
Purpose of Model Review:
Client’s Perspective
To ensure a quality product
To ensure safety
To examine operability
To examine repair/maintenance
To examine environmental impacts
Engineering Company’s Perspective
To meet the client’s requirements
To prevent design errors
To achieve better drawing quality
To follow drawings and construction schedules
To improve earnings and provide client satisfaction
Stages of Model Review:
30% Model review: Review from a plot plan point of view – location, installation, construction, maintenance, and repair of the equipment
60% Model review: Review from an operation point of view – process requirements, tray routes for piping. Also, make appropriate changes from the 30% review. Major Line Routing reviewed and finalized.
90% Model Review: Overall review aimed at producing the final drawings. Make appropriate changes from 30% and 60% Model Reviews.
Required Input for Piping Design: Workflow
Required Input: It ranges from PFD/P&ID, Line List, Data Sheets, and Vendor Prints to the various deliverables of other disciplines like Structural Arrangement Drawings, Cable Layout, Level sketches of Instrumentation, etc.
Piping Activities: Preparing Site Plan, Overall Plot Plan, Unit Plot Plan, Nozzle Orientation, Rack sizing, Equipment/ Piping modeling, Laying outlines, Job instruction, Project Specification, Materials Take Off (MTO), Stress Analysis, etc.
Piping Outputs: Site Plan, Overall Plot Plan, Unit Plot Plan, Nozzle Orientation, Complete 3D model, Isometrics, Pipe MTO, Project Specifications, Piping GA drawings, Special Pipe Support Index/ datasheet, Support MTO, Insulation/ Paint MTO.
It’s an IPO cycle as shown in figure (Fig. 1) below:
Fig. 1: The IPO Cycle
Importance of statutory requirement in piping design
Statutory and Regulatory Requirements in Piping Designs….
When a plant is set up in any country, its design is governed by the statutory and regulatory requirements of that country. Some examples are as follows:
Petroleum and Explosive Safety Organisation (PESO): With an overall objective of ensuring the safety and security of public and property from fire and explosion, PESO as a statutory authority is entrusted with the administration of the Explosives Act, 1884, Petroleum Act, 1934; Inflammable Substances Act, 1952 and other related acts.
Oil India Safety Directorate(OISD): e.g. OISD-STD-118-Layout for Oil & Gas Installations, OISD-STD-117- Fire Protection Facilities for Petroleum Depot/ terminals, etc.
Indian Boiler Regulation (IBR): IBR approval is required for IBR equipment and lines.
Environment /Pollution related Norms: Also to be complied with such as allowable emission rate of NOx, SOx
Internal and External Coordination in Piping design
As the work of piping design tends to overlap with that of Process, Mechanical, Civil/Architecture, Electrical, Instrument engineering teams, and others, it is important for a piping engineerto work in close coordination with related departments all the while maintaining a broad and comprehensive view of the plant.
The process is the upstream discipline for the engineering group. The process provides PFD, Process Data Sheets, P&ID, Equipment List, Line List, etc.
The mechanical group gives us the Mechanical Data Sheet of Equipment. In the later stages of design, Vendor Prints (VPs) are made available.
The Civil/ Structural/ Architectural group provides the required foundation for equipment, structures for racks/ equipment, underground trenches, pits, structures for Miscellaneous Pipe Supports, etc.
The instrumentation group supplies the data for inline and online instruments, control valves, and relief valves.
Electrical gives the hazardous area classification drawings, cable layout, etc.
All these disciplines are also involved in preparing the Plot Plans
The piping design has enormous implications for plant construction, operation, and function.
Construction and Procurement may be an internal/ external group depending on the project scope
The most important external group to be coordinated with is the Client.
There are other external groups such as the Licensor and the PMC. They are equally important. ITB to be thoroughly read to understand the PMC requirement and the Licensor requirement.
Fig. 2: Piping Vs Other Disciplines
Technical and General requirements for Spring Hangers while purchasing
In all critical process and power plant units’ the use of spring hangers are quite common. These are mostly used near equipment to bring nozzle loads within vendor-allowable values. Purchasing a spring hanger must serve the appropriate function of carrying the loads within its working range without failure. To maintain the quality of spring hangers the hangers must meet several technical and general requirements. This article will mention about few such important points. For more information about spring hangers click here to visit my earlier articles on spring hangers.
The following requirements at a minimum must be met by the vendor while delivering the spring hanger supports:
Spring support mechanical design, material & fabrication shall be performed by the vendor according to MSS SP 58 unless specified otherwise in this specification.
Spring support shall meet the requirements of the MSS SP 69, “Pipe Hanger and Supports- Selection and Application” unless specified otherwise in this specification.
Spring support shall meet the requirements of the MSS SP 89, “Pipe Hanger and Supports – Fabrication and Installation Practices” unless specified otherwise in this specification.
Spring support shall be supplied in line with the requirements specified in Datasheet.
Spring support assembly shall be designed to withstand 10000 plus load reversals at maintained consistency.
All parts of the spring support assembly shall be designed to withstand 2 times the operating load or hydro test load, whichever is higher.
All alloy steel springs shall be shot-peened as per supplementary requirement S3 of ASTM A125.
All Hot wound alloy steel springs shall be examined by the magnetic particle as per supplementary requirement S2 of ASTM A125.
Spring shall have a surface protection coating suitable for saline coastal conditions e.g. plastic coating.
Each spring support shall be factory pre-set and locked to the cold load given in Datasheet.
All spring supports shall be provided with a locking device (bolt & nut type) which must be designed to re-lock the spring support at any load position within the range of the model.
All spring support shall be provided with means of positively and firmly locking the piston plate for hydrostatic testing of the pipe.
All “can type” spring supports shall have a Teflon pad on the top slide plate of the spring for temperatures up to 200°C. The thickness of pads shall be a minimum of 10 mm for Teflon and 12.5 mm for graphite. It shall be firmly secured by welding 50mm wide X 8 mm (10 mm in the case of Graphite) stopper plates to the load flange. The additional Teflon pad shall be glued with adhesive.
All springs shall be effectively enclosed in a robust casing constructed of steel which is to be provided with a 50 X 50 mm opening at the lowest point in the casing to drain any rainwater or other garbage/ liquid which may enter.
For lifting large-size spring support (with a dead weight of more than 50 Kg), lifting lugs shall be provided.
All springs shall have an affixed load and travel plate on which hot and cold positions are clearly marked. The positioning plate shall have an integral pointer to indicate the load on the spring at any time. The load may be indicated by punching also.
The manufacturer shall submit a spring support assembly drawing with a hot load, cold load, spring constant, and all the dimensions in detail indicating the overall length.
All spring support shall be delivered as a complete assembly, complete with a blocking device installed.
The Vendor shall comply with the inspection & testing requirements of MSS SP 58.
The Purchaser and Company reserve the right to carry out quality and technical review at both Supplier’s and Sub-Supplier’s works including review of quality documentation.
Test reports for all mandatory (as well as supplementary, wherever specified) tests shall be furnished.
The supplier shall provide a warranty for the material for eighteen (18) months from the date of delivery or twelve months from the date of commissioning whichever is longer.
Materials
The material of springs shall meet the minimum requirements as mentioned in ASTM A125. The material of construction shall be as follows.
Spring housing shall be painted in accordance with approved specifications or standards. All threaded components shall be hot dip galvanized as per ASTM A123.
The following additional information shall be legibly and conspicuously attached to the spring casing:
Project Name
Spring Support Tag
Vendor’s Model No.
Load Scale (N)
Travel Scale (mm)
Hot Load Position (Red indicator)
Cold Load Position (Green indicator)
Spring Rate
Documents to be submitted with tender:
Sketches/catalogs indicating selection criteria for the spring model.
List of materials for spring, housing, and their painting/coating detail.
